In recent years, a reversible thermosensitive recording medium (also called “thermosensitive reversible recording medium” and “recording medium”) has attracted attention, in which an image can temporarily be formed; i.e., the image can be erased when it is unnecessary. A typical known example of the reversible thermosensitive recording medium is a reversible thermosensitive recording medium containing a polymer, a developer (e.g., a phenol compound, an aliphatic carboxylic acid compound or an organic phosphoric acid compound having a long-chain aliphatic hydrocarbon group) and a color coupler (e.g., a leuco dye) where the developer and the color coupler are dispersed in the polymer (see PTLs 1 and 2).
Such a reversible thermosensitive recording medium mainly contains, as a support, a PET film having a magnetic recording layer, and is used as a point card on the market in many cases. In addition, other various reversible thermosensitive recording media have been proposed, which include a thin support, a reversible thermosensitive recording layer provided on one surface of the support, an adhesive layer provided on the other surface of the support and various base materials where the reversible thermosensitive recording layer, the support and the adhesive layer are laminated on the various base materials (see, for example, PTLs 3 to 6).
However, these proposed reversible thermosensitive recording media are cards with limited sizes, since they are combined with an optical memory, a contact-type IC, a non-contact-type IC or magnetic recording and also, most of the base materials are thick. These cards have limited applications. Therefore, they are not suitable to an admission ticket or a sticker for a frozen food container, industrial product, every type of chemical container or the like, or large screen and various displays for physical distribution control, manufacturing process management or the like.
Then, for the aforementioned applications, the thermosensitive reversible recording medium must have a sheet size larger than a card size. Here, the “sheet size” means a size larger than the card size (54 mm×85 mm).
The reversible thermosensitive recording medium used as a sheet has a size larger than that of a point card or a card with a thick base material. Thus, during conveyance in a printer, the reversible thermosensitive recording media of a sheet size tend to be problematically charged as a result of, for example, contact between the reversible thermosensitive recording media and contact between each medium and a conveyance roller. In addition, the reversible thermosensitive recording medium of a sheet size has a larger contact area and thus problematically accumulates a larger amount of electrostatic charges. As a result, in an assembly line of electronic parts or the like, when a paper sheet called an inspection sheet, an operation instruction sheet or a process management sheet is replaced with a sheet of a reversible thermosensitive recording medium having accumulated electrostatic charge which is selected by an operator from the reversible thermosensitive recording media stacked on a discharge tray of the printer, the electrostatic charge of the reversible thermosensitive recording medium problematically destroys products such as electronic parts. Also, the reversible thermosensitive recording media stick to each other due to the accumulated electrostatic charge, making it difficult for them to be fed from a feeding tray of the printer. Furthermore, in each reversible thermosensitive recording medium, the degree of curling, which is due to shrinkage after repetitive printing/erasing with heat, becomes large to problematically cause failure in conveyance.
In view of this, some reports have been presented on a reversible thermosensitive recording medium having an improved antistatic property to solve the above problems.
First, there has been proposed a thermosensitive reversible recording medium which, under 20° C. and 65% RH, has a surface resistance of 1×1013 Ω/sq. or lower and a coefficient of a surface static friction of 0.65 or lower (see PTL 7).
This proposed thermosensitive reversible recording medium, however, shows a lowered surface resistance when measured under low humidity environments. In particular, when the surface resistance is equal to or lower than 1×109Ω/sq., the reversible thermosensitive recording medium cannot sufficiently be charge-eliminated under low humidity environments. As a result, when repetitively printed and erased under low humidity environment, the reversible thermosensitive recording media are charged and stick to each other in the printer, causing problematic failure in conveyance. Also, curling becomes severe after repetitive use and thus causes failure in conveyance in the printer.
Second, there has been proposed a reversible thermosensitive recording medium containing conductive powder having a minor axis of 1 μm or less (see PTL 8).
According to this proposal, dust attached onto the reversible thermosensitive recording medium is reduced. However, this literature neither discloses nor suggests surface conditions of the reversible thermosensitive recording medium. Actually, the surface conditions of the reversible thermosensitive recording medium raise difficulties in conveying the reversible thermosensitive recording medium with a feeding roller, when the reversible thermosensitive recording media are conveyed in a superposed manner in the printer. As a result, the sheets cannot be separated from one another to cause failure in conveyance. In addition, when repetitively printed and erased, the reversible thermosensitive recording medium is curled due to heat applied during printing/erasing, which causes failure in conveyance in the printer.
Third, there has been proposed a reversible thermosensitive recording medium having one or more of a layer containing conductive metallic oxide semiconductor powder which is a conductive pigment coated with tin oxide (see PTL 9).
However, this literature does not describe surface conditions of the reversible thermosensitive recording medium similar to the above literature. Actually, the surface conditions of the reversible thermosensitive recording medium raise difficulties in conveying the reversible thermosensitive recording medium with a feeding roller, when the reversible thermosensitive recording media are conveyed in a superposed manner in the printer. In addition, when repetitively printed and erased, the reversible thermosensitive recording medium is curled due to heat applied during printing/erasing, which causes failure in conveyance in the printer.
Meanwhile, in the field of a thermal transfer image receiving sheet (thermosensitive recording medium), some reports have been presented on examples in which an antistatic property has been improved.
First, there has been proposed a thermal transfer image receiving sheet containing conductive needle crystals (see PTL 10).
However, when this proposed thermal transfer image receiving sheet is used directly as a reversible thermosensitive recording medium, sufficient antistatic effect cannot be obtained. This literature does not describe an example in which an antistatic layer is provided on the uppermost surface thereof. In this case, the thermal transfer image receiving sheet becomes relatively difficult to convey in the printer. Furthermore, during repetitive printing/erasing of this proposed thermal transfer image receiving sheet used as the reversible thermosensitive recording medium, the reversible thermosensitive recording media stick to each other, potentially causing multi feed. Also, curling is not sufficiently prevented from occurring, and the reversible thermosensitive recording medium is progressively curled due to heat applied during repetitive printing/erasing, finally leading to failure in conveyance.
Second, there has been proposed a thermosensitive recoding medium including a back layer containing a conductive polymer and spherical fillers (see PTL 11).
This proposed thermosensitive recoding medium exhibits advantageous effects from the viewpoint of preventing static charge and sticking between the media. However, even when the thermosensitive recoding medium is used directly as a reversible thermosensitive recording medium, such effects of preventing static charge and sticking between the media cannot be sufficiently obtained. Furthermore, during repetitive printing/erasing, this reversible thermosensitive recoding medium involves scratches, and is progressively curled due to heat applied during repetitive printing/erasing, finally leading to failure in conveyance.
To solve the above problems, there has been reported a reversible thermosensitive recording medium having an improved effect of preventing curling.
For example, there has been proposed a reversible thermosensitive recording medium including a protective layer (front surface) and a back coating layer both of which being formed of a UV ray curable polymer, wherein a dynamic friction coefficient is 0.3 or higher between the surfaces of the protective layer and the back coating layer and a dynamic friction coefficient is 0.3 or lower between the surfaces of the protective layers (PTL 12).
This proposed reversible thermosensitive recording medium has an effect of preventing curling. However, when used directly, the reversible thermosensitive recording medium is charged after repetitive printing/erasing. As a result, the reversible thermosensitive recording media stick to each other, causing failure in conveyance. In addition, the surface property of the reversible thermosensitive recording medium changes due to heat and pressure applied by a head as well as heating by an erasing unit during repetitive printing/erasing, causing failure in conveyance. Furthermore, when the reversible thermosensitive recording medium is mistakenly set in the printer such that the front and back surfaces are upset, the difference in friction coefficient is caused between the back surfaces or between the protective layers, causing failure in conveyance.
Also, there is proposed a reversible thermosensitive recording medium having improved antistatic effect and curling preventive effect, which includes a back layer containing conductive needle fillers of titanium oxide coated with antimony-doped tin oxide and a UV ray curable polymer (see PTL 13).
This proposed reversible thermosensitive recording medium has good antistatic effect and curling preventive effect. This literature, however, does not describe prevention of sticking between the media. In the operation sites, the media stick to each other via water, oil or the like, potentially causing multi feed. In addition, antimony is a harmful substance to the environment. Thus, demand has arisen for development of a reversible thermosensitive recording medium formed of a material having less environmental load.
As described above, at present, there have not been provided yet reversible thermosensitive recording media and relevant arts which meet all of the requirements of preventing static charging, curling, sticking between media due to oil, water or the like in use, and scratch formation after repetitive printing/erasing, as well as the requirement of exhibiting excellent conveyance property, although there are some methods of preventing static charging and curling.